Why is SMT (multicore) support hard for Amiga OS?

[NorthWay]

The quick and dirty answer?

Way too many programmers (including the C= library ones) have been too quick and dirty.
Forbid()/Permit() is actually documented as a macro in the official includes. Same with Disable()/Enable().
That means you can't insert your fix in the jumptables and expect all uses of the Big Lock to get caught. Possible solutions are to do Enforcer style ExecBase hacks to mmu-track the lock or interrupt register.

From there on in I bet there are many more pitfalls.

[NorthWay]

Fair enough, but at this point I think we are looking at next-gen amiga systems, and I don't think an application recompile is too unreasonable if required.

Let me rephrase what I said:
The Forbid/Permit semaphore is a publicly readable and writeable kernel structure.
You are free to change the definition of how the OS works, but then you are on your own. AROS (very sensibly (IMNSHO)) decided to take 3.1 with warts and all and implement that first. When they are 100% done you can add New Stuff.